EESS talk on "Catalysis of Fe(III)-Mineral Transformations by Fe(II)"


Event details

Date 20.02.2024
Hour 12:1513:15
Speaker Dr. Kevin Rosso, Pacific Northwest National Laboratory, USA
Location Online
Category Conferences - Seminars
Event Language English
Fe(III)-(oxyhydr)oxides are critical secondary minerals in soils and sediments that control iron bioavailability and the cycling of coupled elements. The nanomineral ferrihydrite is a poorly crystalline and metastable initial form, typically resulting from aqueous Fe(II) oxidation during meteoric recharge of otherwise stagnant suboxic pore water. Because it is metastable, ferrihydrite will spontaneously transform into more crystalline and persistent bulk phases such as goethite or hematite. However, in the absence of a redox catalyst such as Fe(II), this transformation tends to be kinetically hindered, consistent with both 1) the extremely low aqueous solubility of Fe(III), which restricts the dissolution/reprecipitation pathway, and 2) the extremely low solid-state diffusivity of lattice iron expected at room temperature, which restricts the topotactic transformation pathway. Particle-based recrystallization to goethite has been proposed, but this cannot transform ferrihydrite without recruiting one or both of these two slow pathways.  Furthermore, in the presence of millimolar aqueous Fe(II), the goethite product and its physical characteristics are essentially unchanged, but the kinetics are, intriguingly, about 1000x faster.
I will present a comprehensive experimental/computational investigation into this transformation at suboxic conditions and circumneutral pH, resolved at the iron site occupancy level. In the absence of added Fe(II), two-line ferrihydrite aged over two years was periodically characterized using in situ mXRD with detailed line shape analysis, electron microscopy, and synchrotron Fe L-edge and O K-edge XAS/XMCD spectroscopy quantitatively analyzed using MRCI ab initio calculations. The results are compared to DFT calculations of the relative thermodynamic stabilities of several possible ferrihydrite structure models as a function of hydration state, and goethite, along with activation energies for solid-state diffusion of iron along hypothetical topotactic channels. Tetrahedral Fe(III) is clearly resolved in 2 d ferrihydrite, diminishing concomitantly with increasing octahedral Fe(III) and decreasing net magnetic moment until the first appearance of goethite at ~ 100 d. Calculated iron diffusion activation energies are as low as 1 eV per unit cell. In the presence of Fe(II), the accelerated transformation kinetics appear to involve facile mass transfer of a labile Fe(III) pool created on ferrihydrite surfaces during oxidative adsorption of Fe(II).  The findings are being assembled into transformation kinetics models that will help resolve the relative importance of dissolution/reprecipitation vs. solid-state pathways, to lay out the first atomically-resolved mechanism of this important Fe(III)-(oxyhydr)oxide mineral transformation.

Short Biography:
Dr. Rosso is best known for his pioneering research on electron transfer reactions between aqueous ions, mineral surfaces, and bacterial enzymes. Beginning with topics such as metal sulfide oxidation, bacterial reduction of metal oxides, contaminant interactions with clay minerals, and mechanisms of crystal growth and dissolution, his research expanded into geologic carbon sequestration, stress corrosion cracking in alloys, performance optimization of lithium battery materials, and the design of semiconductor materials for solar photocatalysis. Dr. Rosso is well recognized as being at the center of the field of molecular geochemistry, a field he helped create with the inception of advanced tools such as scanning probe microscopy, quantum mechanical molecular simulations, and massively parallel supercomputers.

Practical information

  • General public
  • Free
  • This event is internal


  • EESS - IIE


  • Prof. Rislan Bernier-Latmani, EML


(iron oxide minerals redox crystallization electron transfer